The invention relates to a process for producing moulded parts made of Al or an Al alloy and comprising integrated channels with a view to the distribution of a lubricant or to the circulation of a liquid coolant.
it applies especially to the manufacture of cylinder heads.
There is known a process of this type, which is described in Japanese Patents JA-55-73455 and JA-55-68168 in which a metallic tube filled with sand or a similar material is embedded in a moulded part. After solidification the sand is removed. However, this method has the following disadvantages:
before casting, it is necessary to fill the tubes with sand, which constitutes an additional and tricky operation, especially if the tubes are narrow and tortuous;
after casting, it is necessary to remove the sand contained within the channels, which may be difficult if the channels are narrow and tortuous;
it may not be possible to remove grains of sand remaining caught up on the internal walls of the tube, which may lead to difficulties if these grains are subsequently drawn into the application circuits (for lubricating or cooling);
difficulties in refrigerating the tubes during casting.
The applicant, wishing to obviate these difficulties, has thus developed the following method according to the invention.
One or more main tubes, made of aluminum or an alloy thereof and previously formed and optionally equipped with one or more taps, is/are placed in the mould, in a specified position, the open ends of the tubes being able to correspond with the outside of the mould, either through the latter or through cores; the closed ends may be embedded in the part.
The alloy of the part is then poured into the cavity of the mould after closure of the latter, in order to embed the tube or system of tubes.
The alloy constituting the tubes and their thicknesses are specified in such a manner as to avoid any local melting during the filling of the mould with the liquid alloy. It is generally recommended to use a tube of A5 (AFNOR-NF-57702 Standard) of thickness greater than or equal to 1 mm. The bend radii of the tubes must not generate, in the stretched fibre of the elbows, thickness reductions which are too great and are incompatible with the strength of the tube when filling the mould; preferably the bend radius is greater than or equal to 3 times the external diameter of the tubes.
The taps are fixed on the main tube:
a) either by collars, which are obtained by axial compression of the tube and are formed to the external diameter of the main tube. Fixing then takes placed by adhesive securement or welding. The positioning is given by the collar and the fit of the tap in the main tube (FIG. 2);
b) or by press-fitting the chamfered end of the tap into an orifice of the main tube with a positive oversize ranging up to 0.5 mm. The positioning is given by the end of the longitudinal ribs of the taps ensuring the lateral interference (FIG. 3);
c) or by direct welding.
The depth of penetration of a tap into the main tube is of the order of 1 mm in relation to its internal surface. This positioning is fixed by the position of the collar in case a) or by external longitudinal ribbing, by embossing, of the end of the fitting in case b). In the latter case, the resulting diametral thickening ensures the lateral interference in this case and the end flanges of the ribs ensure the axial positioning of the fitting on the tube. The positioning of the tube (or of the system of tubes) in the mould ensures its centring in relation to its reference position, which allows for axial expansion of the tubes and the rotary blocking of the latter.
The positioning of the tube (or the system of tubes) in the mould is achieved for example by an elbow at 90.degree. external to the useful portion of the moulded part and inserted into a core adjacent to the mould with a clearance of the order of 1 to 2 mm, for an overall length of approximately 50 cm. The other end of the tube is inserted into a cylindrical housing core permitting the free axial expansion of the tube but also ensuring a perfect centring of the system of tubes in the mould. The mould itself is produced either in sand, metal or a mixture, under the usual conditions for the casting of aluminum alloys. The alloys usually employed are AS7G or AS5U3G in the Y20,23 or Y30,33 states in accordance with AFNOR NF-A-57702 Standard.
The dimensions between tap centres will be adjusted, in relation to the desired dimensions, by an amount which takes into account the deformations of the main tube during the casting and the solidification of the part: namely elongation under the effect of the heat of the liquid metal and shrinkage when the part solidifies.
The temperature for casting the part is the lowest possible temperature which makes it possible to obtain correct production of the part and non-melting of the tube. The casting temperature is less than or equal to 760.degree. C. and preferably between 720.degree. C. and 740.degree. C.
By way of indication, the usual internal diameters of the tubes employed are generally between 3 and 15 mm.
It is preferably to choose the alloys for the tube or system of tubes and for the cast part in such a manner that the melting point of the alloy of the moulded part is less than or equal to that of the metal (or alloy) constituting the tube (or system of tubes).
During the filling phase, the contact time between the system of tubes and the molten metal must be less than 10 seconds, and preferably 5 seconds.
The tube (or system of tubes) may also be cooled during the casting operation by a circulation of a cooling fluid, such as compressed air or liquid nitrogen vapour.
The invention will be better understood with the help of the following example, illustrated by FIGS. 1 to 3, reproducing the conditions of casting a part of cylinder head.